A vector controlled PWM voltage source inverter may be used to excite a PM motor, such as an interior permanent magnet (IPM) motor. Using this approach provides accurate torque control, improved dynamic response, and increased motor efficiency. Reducing the magnetic flux inside the motor, which is also called flux or field weakening, may provide improved power characteristics of the PM motor at higher speeds. Flux weakening in a PM motor can be accomplished by adjusting the stator excitation.
During a constant torque region, closed loop current regulators control the applied PWM voltage excitation so that the instantaneous phase currents follow their commanded values. However, saturation of the current regulators may occur at higher speeds when the motor terminal voltage approaches the maximum voltage of the PWM inverter. Beyond this point, the flux should be weakened to maintain proper current regulation up to maximum motor speed.
Conventional field weakening approaches rely on voltage control loops or current angle control loops. Inherently, the voltage control loop approach has poor dynamic performance. Additionally, for IPM machines with reluctance and magnet flux, using the voltage control loop for field weakening does not guarantee optimum torque per ampere in the field-weakening region.
The current angle control loop approach does not work with high back EMF PM machines since it cannot inject any field weakening current when torque is not applied at higher speeds. In addition, for a given constant torque command, the current angle control loop approach will not maintain constant developed torque (i.e. torque linearity) as the drive enters into field weakening and approaches maximum speed.